Mask inspection, or photo mask inspection, is an operation of checking the correctness of the fabricated photo masks (e.g., used for semiconductor device fabrications). Modern technologies for locating defects in photo masks are automated systems that involve scanning electron microscopy and other advanced tools. Existing illumination systems in the market for mask inspection employ ultra-violet light at or above 193 nm that are not sufficient to resolve the features and defects below the 22 nm node. In order to resolve features and defects below 22 nm node, light of shorter wavelength in the EUV (e.g., 13.5 nm) region needs to be used. Since the brightness of commercially available EUV source is not sufficient, multiple sources are required. Some illumination techniques have been disclosed in an attempt to support multiple sources.
One such technique is disclosed in U.S. Pat. No. 6,396,068, which uses a two-stage method to increase the number of source units that can be temporally multiplexed. According to this technique, multiple sources are placed on translational stages that select different sources at different time. Then a rotational stage acts as a beam combiner that selects beams from several sources selected in the first stage. However, it is noted that this technique can only multiplex a few source units within a limited track length (e.g., 2-3 m from a commercially available EUV source to mask) and small range of normal or grazing incident angles where mirror reflectivity is high (R>60% for 0-20° normal incidence, and R>80% for 0-15° grazing incidence). Furthermore, the reflected optical path changes its direction due to the duration of each pulse, the sources time jitter (pulses are emitted at different time than expected), and the velocity instability of rotatable or translational base.
Another technique is disclosed in U.S. Pat. No. 6,861,656, which selectively tilts a planar mirror angle in coordinates with a selective activation of EUV source units. However, it is noted that this technique also can only multiplex a few source units within a limited track length and small range of normal or grazing incident angles where mirror reflectivity is high. Furthermore, the reflected optical path also changes its direction due to the duration of each pulse, the sources time jitter, and the velocity instability of rotatable or translational base.
Still another technique is disclosed in U.S. Pat. No. 7,183,565, which uses a rotatable base to reflect EUV beams from multiple sources. The rotatable base comprises multiple mirrors mounted at various angles and displaced radially from the axis of rotation. The mirrors are positioned to reflect light in the near normal incident direction. However, it is noted that this technique also has the same shortcomings as the other techniques described above.
A further technique is disclosed in U.S. patent application Ser. No. 11/622,241, which uses a reflecting optical element that is mounted to a step or servo rotatable motor to reflect multiple EUV sources to a common optical path for use in semiconductor lithography. It is noted that this technique is only suitable for lithography usage where the dwell time is about 24%, whereas mask inspection requires 100% dwell time. Furthermore, this technique also can only multiplex a few source units within a limited track length and small range of normal or grazing incident angles where mirror reflectivity is high.
Therein lies a need for a method and apparatus for delivering EUV photons from multiple sources to an EUV photo mask for mask inspection, without the aforementioned shortcomings.